How do I get the return value of a function, ran by a pthread?

Question

I have a problem with my C-program and I think I need some help. My program is doing some calculations using multiple threads. Every thread runs a method with only one parameter and in the end returns an integer.

Now, to complete my calculation, it is necessary to take the sum of all sub-calculations, that means the sum of all integers returned by the threads.

But somehow, that result is not correct. I think I made a mistake in getting all returned integers from the threads. Here is my code:

//creating the threads (with splitArray[] as an array of pointers to other arrays)
pthread_t threads[n];
for (int i = 0; i < n; i++) {
  pthread_create(&threads[i], NULL, (void * )(countPrime), (void * )splitArray[i]);
}


//getting the results of the threads
int numPrimes = 0;
int save;
for (int i = 0; i < n; i++) {
    pthread_join(threads[i],(void **) &save);
    numPrimes = numPrimes +  save;
}

This is the method every given to every thread:

 int countPrime(int array[]) {
    int numPrimes = 0;

    for (int i = 0; i < size; i++) {
        //checking if array[i] is a prime number
        if (isPrime(array[i])) {
            numPrimes++;
        }
    }
    return numPrimes;
 }

Have I made a mistake? I am new to C and so I am not really confident about working with pointers, which seems to be necessary in this case.

Thanks a lot :)

What do you really return from the thread? Please try to create a [mcve] to show us. — Some programmer dude, May 11 '20 at 13:44
[Here is your answer](https://stackoverflow.com/questions/2251452/how-to-return-a-value-from-pthread-threads-in-c) also you can check [here](https://docs.oracle.com/cd/E19253-01/816-5137/tlib-24/index.html) — ajayg2808, May 11 '20 at 14:02
What is `splitArray`? What is the result you get? What is the result you expected? What is the input that gives that result? Please take some time to refresh [ask] as well as [this question checklist](https://codeblog.jonskeet.uk/2012/11/24/stack-overflow-question-checklist/). — Some programmer dude, May 11 '20 at 14:07

Some programmer dude · Answer 1 · 2020-05-11T14:18:52.190

The thread function is supposed to be returning a void * pointer, and on a 64-bit machine void * is typically 64 bits wide, while int is typically only 32 bits wide.

When you return a 32-bit value from a function that is expected to return a 64-bit value, what will the extra 32 bits have for value? That's unknown and your code exhibits undefined behavior because of this.

To solve your problem (as I think it might be) you need to update the thread function to return the correct type (void *) and do some casting to make sure the value you return is of the correct type and size:

void *countPrime(int array[]) {
    ...
    return (void *) (intptr_t) numPrimes;
}

Then when you fetch the value, you need to use the correct types as well, and use the opposite casting:

void *result;
for (int i = 0; i < n; i++) {
    pthread_join(threads[i],&result);
    numPrimes = numPrimes +  (int) (intptr_t) result;
}

Note that returning a non-pointer value like this (as well as passing a non-pointer value as argument to the thread function) is almost the only case where most people would agree that such casting is okay. Otherwise, it generally never is.

How do I get the return value of a function, ran by a pthread?

1 Answers1